1. Field of the Invention
The present invention relates generally to an apparatus for digital-to-analogue conversion, which is applicable to, in particular, the output stage of Digital Audio Equipment.
2. Background of the Invention
A system for one-bit digital-to-analogue conversion (D/A conversion) with a Noise Shaping circuit has been developed and widely used in digital audio systems. Referring to FIG. 6, basic construction of the one-bit D/A conversion system will be explained.
In FIG. 6, a digital input signal D.sub.i with N.sub.0 bits derived from an input terminal 21 is provided to a digital filter 22. The signal is processed by over-sampling at the filter 22 and becomes data with N.sub.1 bits. The N.sub.1 bit data is provided to a noise shaping circuit 23 and processed by noise shaping. As a result, the data is compressed to N.sub.2 bits (normally, N.sub.2 equals one to four bits). By means of this noise shaping, quantization noise in a low frequency band is driven to high frequency band so that the signal-to-noise (S/N) ratio within the audible frequency band can be improved. The N.sub.2 bit data is converted into a one bit (binary value) pulse wave at a one bit D/A converter 24. The output of this D/A conversion is provided to a Low Pass Filter 25, and a smooth analogue output signal A.sub.0 is retrieved from an output terminal 26.
In such a D/A conversion system with a noise shaping circuit, a dither (artificial noise) signal is added at the input of the noise shaping circuit to avoid generation of a fixed pattern against low level digital input signals. In other words, in FIG. 6, the adder 27 is connected between the digital filter 22 and the noise shaping circuit 23 so that the dither is provided from the dither generator 28 to the adder 27. The dither from the dither generator 28 is added to the output signal of digital filter 22 and then provided to the noise shaping circuit 23. This prevents a fixed pattern from being generated when the digital input signal has extremely low level.
However, in the above mentioned apparatus for D/A conversion, dither is provided into the noise shaping circuit 23 even when the digital input signal D.sub.i is muted and becomes zero. Then, the output of noise shaping circuit 23 doesn't become zero but has the frequency component of dither. The resulting output of the One-Bit D/A converter 24 has the same dither component. Though the waveform of the output pulse output of the One-Bit D/A converter 24 averages to zero after a long time period, the waveform doesn't always become zero. In other words, the D/A converter 24 does not always output a waveform with 50% pulse duty cycle. This creates a problem of increased noise compared with the cases when the duty cycle is fixed to 50%.
To resolve this problem, it has been proposed that the output waveform from the One-Bit D/A converter 24 is fixed so as to have a 50% duty cycle whenever the digital input signal is muted. However, when the waveform of the output pulse is fixed to have a 50% duty cycle, spike noise is generated. Therefore, the previously mentioned problem is not resolved or invites another problem as mentioned above.